Because of the richness of the information available from NMR, it has often been argued that NMR is the most powerful analytical technique for molecular structure determination. However, NMR has been more successful with liquids or materials dissolved in solvents than with rigid solids. The basic problem in NMR of solids is that rapid molecular tumbling and diffusion are not naturally present to average out chemical shift anisotropy and dipolar couplings of abundant spin nuclides. Hence, the lines are normally broad and unresolved (often hundreds of ppm in width). A large number of techniques have been developed to improve the resolution in NMR of solids, but most modern techniques include extremely rapid spinning of the sample at the “Magic Angle” (the zero of the second Legendre polynomial, 54.7°) with respect to B0. If the rotational rate is fast compared to chemical shift anisotropies and dipolar couplings (in units of Hz), the resolution is dramatically improved—often by two or three orders of magnitude. Even when the spinning is not fast enough to satisfy the above conditions, substantial improvements in resolution are generally obtained from the combination of MAS and multiple-pulse methods.
Many MAS designs have been based somewhat on the classical work of J. W. Beams, and it is customary to refer to conical bearing/drives of the type detailed most clearly in U.S. Pat. No. 4,511,841 as Beams-type Bernoulli axial-bearing/drives. They comprise a rotor conical end (surface on the end of a rotor) of included angle ˜102°, a conical stator surface of included angle ˜91° engaging the rotor conical end, and a number of gas-feed holes through the conical stator surface at compound angles producing gas flow in the annular conical space between the stator and rotor surfaces that is characterized as rotational outward flow. Owing to the converging nature of this conical flow space, the radial component of the flow velocity may be sufficiently high at the periphery for a substantial Bernoulli effect, which, depending on various conditions, may exceed the hydrostatic effects nearer the center. As a result, a stable axial bearing may be formed over a rather wide range of spinning speeds, assuming sufficient space is available near the periphery of the conical surfaces for the gas to exhaust with very low back pressure. It is in fact this requirement which leads to the primary limitation of the utility of Beams-type drives in some important MAS applications, as, for example, when a gradient coil or dewar surrounding the spinner makes it impossible to adequately vent the Beams style bearing/drive.
High-speed NMR MAS spinners can be divided into two general classes: (1) designs that are inherently incompatible with automatic sample change because they require complex, high-precision disassembly/reassembly of the sample spinner for rotor changing, and (2) drop-in designs that are in principle compatible with simple automatic rotor changing. This invention belongs in the second of the above classes.
Some MAS designs in the first of the above categories include the following: In U.S. Pat. No. 4,254,373, Lippmaa discloses a double-ended drive design with no effective provision for either axial stability or high drive efficiency. In U.S. Pat. No. 4,456,882 I disclosed an MAS spinner with single-ended drive using cylindrical, ceramic sample containers with press-fit plastic turbines on hydrostatic air bearings that relies on carefully balanced back pressure from a front cover plate for axial stability against a point bearing. In U.S. Pat. No. 5,202,633 I disclosed a high temperature spinner with a hydrostatic axial bearing formed between the flat bottom end of the rotor and the inward flow exhausting from the radial bearing. Note that at the low flow velocities present here, there is no significant Bernoulli effect. In U.S. Pat. No. 5,508,615, I disclose a method of suppressing whirl instability in the radial bearings at very high surface speeds and improving the stability of balanced axial hydrostatic bearings, similar to the one used in U.S. Pat. No. 5,202,633. In U.S. Pat. No. 6,320,384 B1, an MAS spinner similar to that of U.S. Pat. No. 5,508,615 is used with novel methods of improving rf performance. In U.S. Pat. No. 6,803,764, Hioka discloses a design that incorporates many features of the above inventions. For example, it is worth noting that turbines with number of blades prime to the number of nozzles were in the Doty Scientific 4 mm production model XC4 in 1998, and those 4 mm rotors routinely spin at 25 kHz. The Doty Scientific HS5 production units in 1988 were utilizing blade profiles that resulted in a rotationally rearward velocity component in the turbine exhaust.
In U.S. Pat. No. 4,739,270, Daugaard and Langer (known to be the primary inventor here) disclose an outward-flow conical drive turbine at the top end of the rotor that at first glance seems compatible with automatic rotor change; but in practice it has not been, partially because the larger drive cap diameter complicates the flow requirements for the eject gas and partly because of the extreme sensitivity of this design to back pressure at the plug end (as would arise from a sample eject system) or below the drive turbine. It is worth noting that there is little Bernoulli effect in Langer's design. In U.S. Pat. No. 5,298,864, Muller discloses a laser-heated high-temperature spinner in which the axial bearing is formed on the back side of the drive turbine, again with outward flow. With regard to automation, it suffers the same deficiencies as Langer's design.
Some MAS designs in the second of the above two categories include the following: In U.S. Pat. No. 4,275,350, Hill discloses an attempt (which never succeeded) to achieve a spinner compatible with automatic sample change that includes a modified Beams-type conical drive surface. In U.S. Pat. No. 4,446,430, Stejskal discloses an outward-flow Bernoulli axial bearing formed on a flat end of the rotor and fed from a single, axial hole. In the aforementioned U.S. Pat. No. 4,511,841, Bartuska discloses a modified Beams-type drive; and in his later U.S. Pat. No. 4,940,942, he discloses a method of improving its axial stability and providing variable temperature operation for the sample. In U.S. Pat. No. 6,803,764, Hioka discloses a design of unclear novelty that might appear from FIG. 6 therein, similar to prior art by Bruker, to be compatible with automatic sample change; but FIG. 1 and the text imply that a nozzle cap, similar to that in U.S. Pat. No. 4,456,882, is required for stability, which would make it incompatible with automatic sample change. In “Magnetism in HR NMR Probe Design Part II: HR-MAS,” Concepts in Magnetic Resonance, 10(4) 239–260, 1998, I illustrate in FIG. 7 a modification (which has proved unsuccessful) of the approach by Bartuska.
The axial air bearing of the instant invention utilizes inward flow with minimal rotational component over a rotor conical end to achieve improved stability and stiffness without the need for very-low-back-pressure venting. It is useable at surface speeds from zero to at least 80% of the speed of sound. Moreover, it is readily compatible with automatic sample change.